UO MORPHOLOGY AND EVOLUTION long and very slender, and the interosseous space is long and narrow; they are also long, and bound an extensive space in the Orang. And the African Apes have a wide space between the curved radius and ulna. In the Chimpanzee the head of the radius is circular, and the lower end is massive, prismatic and deeply grooved by the extensor tendons. The roughness for the insertion of the tendon of the biceps is poor, but the interosseous ridge is sharp. The ulna, on the contrary, has no interosseous ridge; its styloid process is very well marked. In the Gorilla the head of the radius is more elliptical, the bicipital tuberosity is better marked, and the cross section of the shaft is pyriform, the narrow part being produced by the interosseous crest. The lower end of the dorsal aspect is grooved by the extensor tendons. The ulna has a convex or reniform surface for articulation with the lower end of the radius. The relative proportions between the length of the forearm and that of the arm have already been described in Chapter IV. Carpus.*—The Gorilla and Chimpanzee have eight carpal bones arranged in two rows as in Man, but the Orang and Gibbons have nine bones, the extra element being termed the "os centrale " or " os intermedium" (fig. 21,B). In the Orang the scaphoid and semilunar bones articulate with the lower end of the radius, the cuneiform is connected to the styloid process of the ulna by a cord-like ligament, and the pisiform bone is large. * The carpus lias been described in papers 10. 42. 95, 122, 162, 217.